Domestically and internationally, antitumor agents including 5-fluorouracil (hereinafter referred to as 5-FU), cisplatin, irinotecan, docetaxel, a combination drug of tegafur/uracil (product name: UFT®), a combination drug of tegafur/gimeracil/oteracil potassium (product name: TS-1®, hereinafter, a preparation comprising tegafur/gimeracil/oteracil potassium at a molar ratio of 1:0.4:1 is referred to as TS-1) are adopted in clinical chemotherapies to treat advanced gastric cancers both solely or in a combination of two or more. Although combination chemotherapy of 5-FU and cisplatin is used as a standard therapy in Europe and the U.S., its life-prolongation effect is still not fully satisfactory. Therefore, combination chemotherapy of TS-1 and cisplatin is attracting attention with the expectation that it can promise a superior life-prolongation effect (Non-patent Literature 1 and 2).